1) Field of the Invention
This invention relates generally to apparatuses and methods for combusting carbonaceous fuels to generate power and more specifically to such apparatuses and methods for increasing efficiency and reducing pollutant combustion products, such as NOx and SOx.
2) Description of Related Art
The combustion of carbon-based compounds, or carbonaceous fuels, is widely used for power generation. In one typical generation system, a carbonaceous fuel, such as natural gas, is mixed with air and combusted in a combustion chamber. The resulting combusted gas is discharged to, and used to rotate, a turbine, which is mechanically coupled to an electric generator. Each system is characterized by a plant net thermal efficiency, or range of plant net thermal efficiency, which is computed according to a ratio of the net useful energy to the thermal energy of combustion. The net useful energy is the useful energy output, e.g. electricity, less the energy inputs, such as the energy required to power the pumps, compressors, and the like. The thermal energy of combustion is the thermal energy generated by the combustion of the combustion fuel.
The carbonaceous fuel used for combustion often includes nitrogen and/or sulfur, which can occur individually or in compounds. Nitrogen is also a major component of the air used for combustion. Thus, the typical combustion process combusts nitrogen and/or sulfur and generates one or more nitrous oxides, collectively referred to as NOx, and sulfur oxides, collectively referred to as SOx. NOx and SOx formed as products of combustion are a major source of atmospheric pollution, and the reduction of these pollutants has been recognized as an important task.
A number of techniques and devices have been employed to prevent the production or discharge of such pollutants during power generation. For example, some final exhaust gases are passed through filters, scrubbers, or converters. Such devices remove some of the pollutants, but the devices are expensive and may reduce the efficiency of the generation process. Also, these devices are not completely effective, so some of the pollutants pass through these devices and reach the atmosphere.
The formation of NOx is significantly reduced by limiting the combustion temperature to below about 2780xc2x0 F. Therefore, one proposed solution to minimize the production of NOx is to limit the temperature of the fuel during combustion. The combustion temperature can be reduced by injecting water or steam into the combustion chamber to cool the gases. However, high temperatures typically still result in localized regions of the combustion chamber, and NOx thus results. Additionally, some NOx is produced even at lower temperatures. Thus, this method alone has not proven fully satisfactory.
A different method of controlling the combustion temperatures, and thus minimizing the formation of pollutants during combustion of carbonaceous fuels, is to modify the ratios of fuel and air to create a rich (over-fueled) mixture or a lean (under-fueled) mixture. For example, a two-stage combustion process can include a first rich combustion zone and a second lean combustion zone. The so-called sub- and super-stoichiometric ratios of air to fuel generally limit the combustion temperatures, but localized regions of high temperatures may still occur where stoichiometric ratios of fuel and air combust. Additionally, as in the water- and steam-cooled systems described above, some amounts of NOx are formed even at the lower combustion temperatures.
Another method of reducing pollutants is described in U.S. Pat. No. 5,715,673 to Beichel. In one embodiment, a hydrocarbon fuel and oxygen are burned in stoichiometric quantities in the presence of water and the resulting gaseous mixture is used to drive three power turbines. Because the fuel and oxygen combustion reactants include little or no nitrogen, virtually no NOx is formed. Undesirably, however, the plant net thermal efficiency of this process is limited to about 44% to 48%. Additionally, the stoichiometric combustion lacks the flexibility of providing useful by-product fuels such as hydrogen and methanol.
Thus, there exists a need for a power generation apparatus and method for combusting carbonaceous fuel without producing NOx and SOx. The apparatus and method should be highly efficient and not overly complex, so as to minimize size, likelihood of failure, and initial and maintenance costs. Preferably, the plant net thermal efficiency should exceed 50%. Finally, the apparatus and method should be versatile so that varying amounts of electricity and other useful products can be generated according to changing needs.
A low-emission, staged-combustion power generation system and associated method for generating power are provided. The power generation system and method of the invention provide for the generation of power, for example, in the form of electricity, without the formation of polluting nitrous oxides (NOx) and sulfur oxides (SOx) by combusting a carbonaceous fuel, such as methane, synthesis gas, or biomass fuels, with an oxidizing fluid. Both the fuel and the oxidizing fluid are substantially free of nitrogen and sulfur. The power generation system and method are efficient due, in part, to a multi-staged combustion in which the carbonaceous fuel is partially combusted before passing through a first power take-off device and subsequently reheated and passed through one or more additional power take-off devices. A sub-stoichiometric rate of oxygen ensures that the initial combustion is a partial combustion of the carbonaceous fuel. Additionally, the exhaust gases from one or more of the power take-off devices can be extracted and processed to provide quantities of useful products such as hydrogen, methanol, steam, carbon dioxide, and other hydrocarbons.
According to one aspect of the present invention, a method of generating power is provided. According to this aspect, a carbonaceous fuel, such as methane, is supplied to a gas generator. A first oxidizing fluid is supplied to the gas generator at a sub-stoichiometric rate to produce an equivalence ratio greater than 1.0, i.e., a fuel rich mixture. For example, the first oxidizing fluid may be supplied to the gas generator at a sub-stoichiometric rate of between about 0 and 50 percent to produce an equivalence ratio above 2.0. The carbonaceous fuel is combusted with the first oxidizing fluid in the gas generator, producing a combusted gas. In one embodiment, at least 98 percent of the carbonaceous fuel is at least partially combusted in the gas generator to form, for example, steam, carbon dioxide, between about 12 and 22 percent hydrogen by volume, and between about 3 and 7 percent carbon monoxide by volume. The combusted gas is discharged to a first power take-off device, and then to a reheater, where it is combusted with a second oxidizing fluid to form a reheated gas. In one embodiment, the combustion in the reheater heats the combusted gas to at least 2000xc2x0 F. to form the reheated gas. The reheated gas can include steam, carbon dioxide, between about 3 and 10 percent hydrogen by volume, between about 1 and 3 percent carbon monoxide by volume, and substantially no nitrogen or sulfur. The reheated gas is then discharged to a second power take-off device. The power take-off devices may be turbines that are coupled to at least one electric generator, which is rotated to generate electricity. Water can be removed from the reheated gas by passing the gas through at least one condenser and at least one compressor. The reheated gas may also be discharged to a catalytic shift reactor to convert carbon monoxide in the gas to hydrogen and carbon dioxide, a separator to separate carbon dioxide, and a carbon monoxide catalytic converter to convert the reheated gas to methanol and hydrogen.
The carbonaceous fuel and oxidizing fluids, which may be generated by separating oxygen from air, are substantially free of nitrogen and sulfur. Hence, the process produces no appreciable amounts of NOx or SOx.
According to another aspect of the invention, a variable portion of the combusted gas from the first power take-off device is discharged to a catalytic shift reactor. The catalytic shift reactor converts carbon monoxide in the variable portion of the combusted gas to hydrogen and carbon dioxide. The variable portion of the combusted gas from the catalytic shift reactor can then be discharged to a separator to separate carbon dioxide from the combusted gas. In another embodiment, a variable portion of the reheated gas from the second power take-off device is discharged to a low pressure reheater. The reheater combusts the variable portion of the reheated gas to form a twice reheated gas and discharges the twice reheated gas to a third power take-off device.
According to yet another aspect, the invention provides a method of generating power, including generating an oxidizing fluid substantially free of nitrogen and sulfur, supplying a carbonaceous fuel substantially free of nitrogen and sulfur to a gas generator, and supplying the oxidizing fluid to the gas generator at a sub-stoichiometric rate relative to the carbonaceous fuel. The carbonaceous fuel is combusted with the oxidizing fluid in the gas generator to produce a combusted gas, and the combusted gas is discharged to a first power take-off device. A first variable portion of the combusted gas is then discharged to a high pressure catalytic shift reactor, and a second variable portion of the combusted gas is discharged to an intermediate pressure reheater. The second variable portion of the combusted gas is combusted with an oxidizing fluid in the intermediate pressure reheater to form a reheated gas, and the reheated gas is discharged to a second power take-off device. Further, a first variable portion of the reheated gas is discharged to an intermediate pressure catalytic shift reactor, and a second variable portion of the reheated gas is discharged to a low pressure reheater, where the reheated gas is combusted with the oxidizing fluid to form a twice reheated gas. The twice reheated gas is discharged to a third power take-off device and a low pressure catalytic shift reactor. Finally, carbon dioxide, hydrogen, and water are separated from the first variable portion of the combusted gas, the first variable portion of the reheated gas, and the twice reheated gas.
The present invention also provides a power generating system that includes sources of a carbonaceous fuel, such as methane, and an oxidizing fluid, both of which are substantially free of nitrogen and sulfur. A gas generator is configured to receive the carbonaceous fuel and the oxidizing fluid and combust the carbonaceous fuel with the oxidizing fluid to produce a combusted gas. In one embodiment, the gas generator is capable of at least partially combusting at least about 98 percent of the carbonaceous fuel. A regulation system is configured to regulate the flow of the oxidizing fluid into the gas generator at a sub-stoichiometric rate relative to the carbonaceous fuel. A first power take-off device, such as a turbine, is configured to receive the combusted gas from the gas generator, and a reheater is configured to receive and combust the combusted gas from the first power take-off device with the oxidizing fluid to form a reheated gas. The reheater may include a partial catalytic bed to facilitate the reaction of hydrogen in the combusted gas with oxygen and may be capable of heating the combusted gas to at least 2000xc2x0 F. In one embodiment, the reheated gas comprises steam, carbon dioxide, between about 3 and 10 percent hydrogen by volume, between about 1 and 3 percent carbon monoxide by volume, and substantially no nitrogen or sulfur. A second power take-off device, such as a turbine, is configured to receive the reheated gas from the reheater. At least one generator is coupled to the first and second power take-off devices. The power generation system may also include a catalytic shift reactor configured to receive the reheated gas from the second power take-off device, at least one condenser and at least one compressor configured to receive the reheated gas from the second power take-off device, and a separator configured to receive the reheated gas from the second power take-off device and capable of separating carbon dioxide from the reheated gas. The power generating system can include an air separation plant for producing the oxidizing fluid from air, and the separator may be configured to receive cryogenic nitrogen from the air separation plant.
In another embodiment, the power generation system includes a carbon monoxide catalytic converter configured to receive the reheated gas from the second power take-off device. A catalytic shift reactor may be configured to receive a variable portion of the combusted gas from the first power take-off device and convert the combusted gas to hydrogen and carbon dioxide. A separator may also be configured to receive the variable portion of the combusted gas from the catalytic shift reactor and separate carbon dioxide from the combusted gas. Additionally, the power generation system may include a low pressure reheater configured to receive and combust a variable portion of the reheated gas from the second power take-off device to form a twice reheated gas and discharge the twice reheated gas to a third power take-off device.
The present invention also provides a power generating system including sources of a carbonaceous fuel and an oxidizing fluid, both substantially free of nitrogen and sulfur. A gas generator is configured to receive the carbonaceous fuel and the oxidizing fluid and combust them to produce a combusted gas. A regulation system is configured to regulate the flow of the oxidizing fluid into the gas generator at a sub-stoichiometric rate relative to the carbonaceous fuel. A first power take-off device is configured to receive the combusted gas from the gas generator. A high pressure catalytic shift reactor is configured to receive a first variable portion of the combusted gas from the first power take-off device, and an intermediate pressure reheater is configured to receive and combust a second variable portion of the combusted gas from the first power take-off device with the oxidizing fluid to form a reheated gas. A second power take-off device is configured to receive the reheated gas from the intermediate pressure reheater. An intermediate pressure catalytic shift reactor is configured to receive a first variable portion of the reheated gas from the second power take-off device, and a low pressure reheater is configured to receive and combust a second variable portion of the reheated gas from the second power take-off device with the oxidizing fluid to form a twice reheated gas. A third power take-off device is configured to receive the twice reheated gas from the low pressure reheater. A low pressure catalytic shift reactor is configured to receive the twice reheated gas from the third power take-off device. At least one separator is configured to receive the first variable portion of the combusted gas, the first variable portion of the reheated gas, and the twice reheated gas and to separate carbon dioxide, hydrogen, and water from the gases. At least one generator is coupled to the first, second, and third power take-off devices.
Thus, the present invention provides a power generation apparatus and method that satisfy the needs of the prior art. The staged combustion does not produce polluting NOx and SOx, and the power generation system and method are highly efficient. Additionally, the initial combustion is a partial combustion, allowing the generation and extraction of useful by-products.